CIVILIZATION AND VEGETATION 333 



nize and remove the cause of disease. Between gas in the soil and gases 

 in the air urban trees lead a more or less poisoned and morbid existence, 

 appreciated and enjoyed only by those who can not or will not go where 

 trees grow rapidly and well, solitary or gregarious, according to their 

 kinds, a delight to the eye, an inspiration to the spirit, a shelter to the 

 birds, and a satisfaction to the lumberman. 



Perhaps this seems an exaggerated statement of urban conditions, 

 but an examination of the trees in the parks of Pittsburgh, Cincinnati, 

 St. Louis and Chicago, among the industrial cities using mainly soft 

 coal, and of New York and Boston, in which hard coal is the principal 

 fuel, will convince the fair-minded observer that something is wrong. 



Air and soil analyses and experiments under controllable conditions, 

 will alone reveal the cause of the condition of urban trees and of those 

 growing near industrial plants from which poisonous fumes emanate. 

 Such analyses of air and soil can be made only by a skilled chemist, and 

 such experiments should be attempted only when a skilled chemist and 

 a trained biologist can cooperate under such conditions of climate and 

 equipment as to justify the expenditure of time and money. 



In the remarkably pure air of Stanford University, with the nearest 

 domestic coal fire a quarter mile away and with gas supplied to the 

 university by a single small main, a greenhouse has been so con- 

 structed that it is completely divided into two exactly similar halves, in 

 which experiments and their controls can be carried on under as nearly 

 natural conditions as can be conceived. In each half of this house there 

 were boxes of growing grain, a considerable variety of potted seedling 

 trees, one, two and three years old, and other plants, aquatic and terres- 

 trial. The grain was grown on the spot from seeds ; the trees were grown 

 from the seed in the pots and were brought into the house so long before 

 they were used in experiments, that they were quite accustomed to their 

 surroundings. Into one half of the experiment house, a dose of sulphur 

 dioxide equal to the proportion in pure city air, was introduced daily 

 (Sundays excepted) for months, during the resting period only for one 

 set of trees, during the growing period only of another set, and during 

 the resting and most of the growing period of a third set. 



The trees continuously in the side of the house dosed daily with a 

 very minute quantity of sulphur dioxide gas, blown into and thoroughly 

 mixed with the air of the house, showed a growth in length two thirds 

 of that of the trees in the pure air of the other half of the house. There 

 are no visible injuries; only measurement shows that those trees in air 

 purer than that of most cities do not thrive, do not make as much 

 growth as trees in thoroughly pure air. Analyses show that the sulphur 

 dioxide is fixed, mainly in the leaves, and thus disturbs the vital 

 activities of plants. The results with grain correspond. These results 

 obtained by experiment justify, therefore, the assertion that the problem 



